List InstanceNotFound as a client exception

get_instance_nw_info() in nova.network.manager should list
InstanceNotFound as a client exception for rpc. This to prevent the
rpc layer from logging a traceback when this exception is raised.
This is an expected type of exception that should be left to the
client to handle.

Resolves bug 1157411.

Change-Id: I6735fb8a386f2256ac8af0ce7fdb36ef7a00b6ce